<sect1 id="ai-cequator">
<sect1info>
<author><firstname>Jason</firstname> <surname>Harris</surname> </author>
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<title>The Celestial Equator</title>
<indexterm><primary>Celestial Equator</primary>
<seealso>Equatorial Coordinates</seealso>
</indexterm>
<para>The <firstterm>Celestial Equator</firstterm> is an imaginary <link linkend="ai-greatcircle">great circle</link> on the <link linkend="ai-csphere">celestial sphere</link>. The celestial equator is the fundamental plane of the <link linkend="equatorial">Equatorial Coordinate System</link>, so it is defined as the locus of points with Declination of zero degrees. It is also the projection of the Earth's equator onto the sky. </para>
<para>The Celestial Equator and the <link linkend="ai-ecliptic">Ecliptic</link> are set at an angle of 23.5 degrees in the sky. The points where they intersect are the Vernal and Autumnal <link linkend="ai-equinox">Equinoxes</link>. </para>
</sect1>
